The present invention relates to electromagnetic shielding panels, and in particular, translucent panels for shielding against electromagnetic interference and which incorporate electroconductive coatings to attenuate electromagnetic radiation.
It is well known that glass and transparent plastics are translucent to short wave electromagnetic radiation and as a consequence such radiation can pass to and from buildings and/or electronic apparatus, such as computers and word processors, in buildings, and/or through protective visual panels on the apparatus. This is considered quite undesirable from an information security aspect as such radiation is readily detectable.
An effective solution to this problem has been proposed whereby all such electronic apparatus is accommodated within a completely electromagnetically shielded enclosure. Such an enclosure usually takes the form of a room without any windows in which the walls and door incorporate a mesh or metal foil which is electrically connected to earth. In such an arrangement, known as a Faraday cage, electromagnetic radiation neither leaves nor enters the room.
It is not always possible, however, for economic and/or aesthetic reasons, for example, to place all electronic apparatus processing sensitive information within such rooms or enclosures. Various alternative proposals are known whereby translucent shielding panels may serve as windows. These panels employ, amongst other things, electrically conductive coatings which are directly electrically connected to earth. Such panels inhibit the transmission of electromagnetic radiation.
For instance, patent specification GB-B-2 064 629 discloses not only an electromagnetic shielding panel which comprises glass panes which incorporate an electroconductive coating but also glass panes which incorporate a fine electrically conductive woven mesh.
To obtain the required attenuation of the electromagnetic radiation, it is essential that the coating and the mesh are connected to earth and this is achieved by means of direct physical electrical connections to both the coating and the mesh.
A disadvantage with this type of panel is that visibility through it is impaired by the conductive mesh.
In another shielding panel arrangement disclosed in patent specification EP-A-222151 improved visibility is obtained. The shielding panel comprises a pair of spaced glass panes on opposing sides of a pair of spaced parallel electrically conductive transparent films mounted in a taut condition in the space between the panes. This panel also has the requirement to electrically connect both the electrically conductive films to earth by a direct physical electrical connection.
In a further shielding panel arrangement disclosed in patent specification GB-A-2 205 527 the panel comprises two electro-conductively coated glass panes arranged in opposed parallel spaced relationship with the coatings facing inwardly towards each other. This shielding panel has the advantages that it provides good attenuation of electromagnetic radiation while additionally providing good optical qualities. Nevertheless, there remains the requirement to make electrical connection to the coatings directly with a physical electrical connection for connection to earth. In this arrangement this is achieved by means of electrically conductive wire mesh making direct electrical contact with the coatings themselves.
Electromagnetic shielding panels fall into several general categories i.e. they may be of the double glazed type with air or gas between two spaced apart parallel panes of glass or they may be of the laminar type with a plastics interlayer sandwiched between two parallel panes of glass. They may even be a combination of both these types.
Generally, where an electroconductive coating or transparent film is employed and because of the delicate nature of such coatings or transparent films, there is a need to protect them from damage. Accordingly such coatings or transparent films usually face inwardly within the panel and may, for example, be protected by a glass outer pane.
As a consequence of this type of construction, there is always a problem of making good direct physical connnection to the inner coating or transparent film which may in certain circumstances prove quite difficult and be expensive in the manufacturing process. For example, in the double glazed constructed units the connection means may adversely impair the environment seal.
An aim of this invention is to provide an electromagnetic shielding panel which overcomes the aforementioned disadvantages, provides effective attenuation to electromagnetic radiation and which also has good optical qualities.